胞外钙对豚鼠耳蜗Deiters细胞钾电流的调制

Extracellular calcium modulates the whole cell potassium currents in Deiters cells isolated from guinea pig cochlea

为观察胞外钙对豚鼠耳蜗单个离体Deiters细胞钾电流的调控作用并探讨其机制,实验记录了:Deiters细胞在正常细胞外液和无钙外液中的全细胞钾电流(whole cell K+ currents,Ik),并分析了其电生理学特性的改变。结果观察到,Deiters 细胞与在正常细胞外液中相比,在祛除细胞外液中的Ca2+后,IK电流幅值明显增加,弦电导值亦明显增加,但其平衡电位未明显改变。在无钙外液中,Ik电流的反转电位向超极化方向明显移位,更接近于按照Nernst方程得出的K+理论平衡电位;而且其稳态激活曲线亦向超极化方向明显移位,但其激活趋势与正常相比无明显改变。此外,观察了Deiters细胞中钙抑制性钾电流的电流-电压关系和电导-电压关系,发现两者均呈“S”形,提示此钙抑制性钾电流可能存在2种不同的钾电导成分。由此,推测可能有两种机制参与胞外钙对Deiters细胞钾电流的调控:(1)Deiters细胞中的IK通道可能存在一个Ca2+敏感结构域,胞外Ca2+可能通过改变此结构域而对IK电流产生调制;(2)Deiters细胞中可能存在一种新型的双相门控性钾通道或钾通道耦联型受体或是一种新型的钾通道亚型,祛除胞外Ca2+可激活此新型钾电导而对IK电流产生调制。由此推测,在听觉形成过程中,胞外钙浓度下降可以对Deiters细胞的全细胞钾电流产生调制,从而更有利于Deiters细胞内K+外流,进而有效地缓冲外毛细胞周围的K+浓度;而且还可以使Deiters细胞产生更快的复极化并有利于维持其静息状态。

To study the modulatory effect of extracellular calcium on the whole cell K+ currents (IK) in isolated Deiters cells, the whole cell K+ currents were recorded when Deiters cells bathed in normal physiological solutions and calcium-free saline, respectively. The electrophysiological characteristics of IK currents were then analyzed with the patch clamp technique. Removing extracellular calcium significantly enhanced the amplitude of the IK currents, which increased by 70.2% at +50 mV test pulse. The chord conductance, measured at -30 mV test pulse, also significantly increased from (3.31 + 3.08) ns (n=42) in the normal solutions to (10.81 ?6.01) ns (n=42) in the calcium-free solutions, whereas, the zero current potential of the IK currents remained unchanged. In calcium-free solutions, the reversal potential of the IK currents was shifted to the direction of hyperpolarization, which was very close to the equilibrium K+ potential based on the Nernst equation. In addition, both the steady state activation curve and the half activation potential, with the averaged value at (-10.13±5.64) mV (n=42), were shifted to the negative. However, the tendency for activation (slope conductances) was the same as that in the normal solutions. Interestingly, both the I-V and the G-V functions deduced from the calcium-inhibited K+ currents in Deiters cells were 'S' shape, implying that at least two different kinds of K+ conductance were involved in this calcium-inhibited K+ currents. In summary, we hypothesize that there are two mechanisms for this modulation: one is that the IK channels in Deiters cells containing a specific calcium sensitive domain, by which extracellular calcium modulates the structure of the K+ channels and then the IK currents; the other is a novel double gated K+ channel or an ionotropic receptor coupled to K+ channels or a new subtype of outward K+ channels. Removing extracellular calcium activates this novel conductance and then modulates the IK currents. These results indicate that a decrease in extracellular calcium not only facilitates the efflux of K+ out of Deiters cells but also accelerates the repolorization by enhancing the IK currents, which in turn can effectively buffer the K+ concentration around the outer hair cells and maintain the resting membrane potential of Deiters cells.